Start at the hill to the left of the entrance to Turffield, and talk to the girl near the top of it to begin the riddle.
Go the middle of the town, and interact with the set of three stones holding up other stones. This is the Grass stone, and it's the first part you need to interact with.
After this, interact with the Water stone, which is on a path to the left of the hill. You should see three stones, with two of them holding up more stones.
Finally, go to the Fire stone, which is the largest one. It's found near the opening to the river going through Turffield, and it should be easy enough to find. Interacting with it will end the challenge and reward you with an Expert Belt.
Source
Hope I helped!
